The present invention relates to a shadow mask type color cathode ray tube, and more particularly to a shadow mask type color cathode ray tube provided with a shadow mask free from occurrence of deformation and reduction in mechanical strength of its apertured portion when the skirt portion of the shadow mask is fitted in its support frame, by reducing curls occurring in the skirt portion of the press-formed shadow mask.
Generally, a color cathode ray tube has a phosphor screen formed on an inner surface of a faceplate of a panel portion of an evacuated envelope, a shadow mask closely spaced from the phosphor screen and suspended within the panel portion, an in-line type electron gun housed in a neck portion of the evacuated envelope, a deflection yoke mounted around a funnel portion of the evacuated envelope.
In operation, three electron beams projected from the in-line type electron gun are deflected in desired directions, pass through one of a large number of electron beam apertures in the shadow mask and impinge upon phosphor picture elements of the corresponding colors of the phosphor screen, respectively, to produce a desired color image on the phosphor screen.
The shadow mask used in the prior art color cathode ray tube has a generally rectangular apertured portion having a large number of electron beam apertures, a generally rectangular window-frame-like imperforate portion surrounding and integral with the apertured portion and a skirt portion bent back from a periphery of the imperforate portion, and the apertured portion, the imperforate portion and the skirt portion are integrally press-formed. The skirt portion is fitted into a support frame and is spot welded at its fitted portions to the support frame to form a shadow mask assembly. The support frame is fixed to the inner sidewall of the panel portion such that the shadow mask opposes the phosphor screen.
FIGS. 4A to 4C illustrate the structure of an example of a prior art shadow mask for a color cathode ray tube, FIG. 4A being a plan view, FIG. 4B being a perspective view of weld points and their vicinities in the skirt portion and FIG. 4C being a cross-sectional view of a portion of the shadow mask ranging from the imperforate portion to the skirt portion. In FIGS. 4A to 4C, reference numeral 41 denotes the shadow mask, 42 is the apertured portion, 43 is the imperforate portion, 44 is the skirt portion and x marks indicate weld points.
The shadow mask 41 has a large number of electron beam apertures (not labeled), a generally rectangular and curved apertured portion 42, a generally rectangular window-frame-like imperforate portion 43 surrounding and integral with the apertured portion 42 and having a curvature in cross section similar to that of the apertured portion 42, and a skirt portion 44 bent back from a periphery of the imperforate portion 43, and the apertured portion. Usually the apertured portion 42, the imperforate portion 43 and the skirt portion 44 are integrally formed by press-forming a thin sheet metal mask blank having the apertured portion.
The mask blank having an apertured portion is very thin and relatively weak in mechanical strength, and therefore the shape of the press-formed shadow mask 41 is not necessarily satisfactory. Especially, since the skirt portion 44 is formed by being bent back at approximately right angles from the periphery of the imperforate portion 43, the skirt portion 44 of the shadow mask 41 curls outwardly by a maximum distance S from a straight line passing through a bend line between the imperforate portion 43 and the skirt portion 44 and parallel to the longitudinal axis of the cathode ray tube, in a region centering about the center of each side of the generally rectangular shadow mask 41, as shown in FIG. 4C.
The press-formed shadow mask 41 is fixed by fitting its skirt portion 44 into the support frame (not shown) and then spot welding the skirt portion 44 to the support frame at several points. The weld points of the skirt portion 44 and the support frame are distributed two approximately at the center of each long side, two approximately at the center of each short side, and one at each corner of the shadow mask 41, for example, as indicated by X marks in FIG. 4B.
As explained above, in the case of the prior art shadow mask 41, occurrence of curls in the skirt portion 44 could not be avoided when the mask blank is press-formed into a unitary mask structure. If the maximum size S of curls is excessive, problems in that it becomes difficult to fit the skirt portion 44 into the support frame and also difficult to subsequently spot weld the fitted parts of the skirt portion 44 to the support frame, resulting in the degradation of workability in fixing the shadow mask 41 to the support frame.
Also in the case of the prior art shadow mask 41, if the skirt portion 44 having curls of the excessive maximum size S is forced to fit into the support frame, stress caused to the skirt portion 44 is transmitted to the apertured portion 42 via the imperforate portion 43, and consequently the curvature of the apertured portion 42 is sometimes deformed and color selecting property of the shadow mask 41 is deteriorated, and also the mechanical strength of the shadow mask 41 is reduced.
The present invention was made against such technical background, and it is one of the objects of the present invention to provide a color cathode ray tube provided a shadow mask capable of improving workability in the operation of fitting the skirt portion of the shadow mask into the support frame and capable of preventing deformation of the curvature of the apertured portion of the shadow mask by reducing the size of curls occurring in the skirt portion.
To accomplish the above object, in accordance with an embodiment of the present invention, there is provided a color cathode ray tube comprising an evacuated envelope comprising a panel portion, a neck portion, and a funnel portion for connecting the panel portion and the neck portion, a generally rectangular shadow mask having a curved apertured portion having a multiplicity of electron-transmissive apertures, a curved imperforate portion surrounding and integral with the apertured portion and a peripheral skirt portion, a generally rectangular support frame for suspending the shadow mask by spot welding the peripheral skirt portion thereto, within the panel portion, the peripheral skirt portion being fitted in the support frame, a phosphor screen formed on an inner surface of the panel portion, an electron gun housed in the neck portion, and a deflection yoke mounted around a vicinity of a transitional region between the funnel portion and the neck portion for scanning electron beams from the electron gun on the phosphor screen, wherein the shadow mask which is made of Invar (an iron-nickel alloy) is provided with a descending step portion extending along an edge of the imperforate portion, the peripheral skirt portion is formed by being bent back from an entire outer edge of the descending step portion, the peripheral skirt portion is provided with a plurality of beads extending in a direction of a height of the peripheral skirt portion and protruding inward in each side of the peripheral skirt portion, and the descending step portion satisfies one of following three conditions (a)-(c); (a) a height H of the descending step portion is in a range of 30% to 50% of H0; (b) a width W of the descending step portion is in a range of 65% to 80% of W0; and (c) a height H and a width w of the descending step portion are in a range of 30% to 50% of H0 and in a range of 65% to 80% of W0; respectively, H0 and W0 being a height H and a width W of the descending step portion at centerlines of long and short sides of the peripheral skirt portion, respectively.
In the present invention, since there is formed a plurality of beads extending in a direction of the height of the skirt portion and protruding inward in each of four sides of the skirt portion, in the operation of press-forming a mask blank into a unitary shadow mask structure, the maximum size of curls occurring in the skirt portion is reduced. Since there is also formed the descending step portion extending along the outer edge of the imperforate portion, even if great stress is caused to the skirt portion after the skirt portion has been fitted into the support frame and the great stress is transmitted to the imperforate portion from the skirt portion, the descending step portion relieves or blocks this great stress such that the great stress is not transmitted to the apertured portion and the curvature of the apertured portion is prevented from being deformed.